Hundreds of bus drivers went on strike Thursday in several Phoenix suburbs, shutting down service that serves some 57,000 riders daily in the affected suburbs and on routes to other cities.

The Valley Metropolitan regional Transit Authority said the strike affects service in Chandler, Gilbert, Mesa and Tempe, as well as express routes from those cities to Phoenix and Scottsdale.

The strike by a union that represents nearly 400 drivers doesn't affect bus service in Phoenix and other suburbs or Valley Metro's light train system.

This is the aftermath of a labor dispute between the Amalgamated Transit Union and First Transit, a company that operates part of Valley Metro's system.

The union said drivers rejected the company's proposal because it would increase their health care contribution and weaken contract provisions on job security.

First Transit said it regrets the strike's inconvenience for passengers and said it made several offers that were fair and equitable "especially considering current economic conditions."

The company's spokeswoman Jen Biddinger said they will contact a federal mediator to determine why the union rejected a last-minute proposal by the First Transit.

Scott Somers, a Mesa City Council member and Valley Metro's board chairman, called the strike unacceptable and "devastating" to riders who use bus service to get to work, school and medical appointments. "It's especially distressing in this economic condition," he said.

The routes shut down by the strike represent approximately 40 percent of Valley Metro's route system.

Valley Metro said riders should seek out carpools, telecommute if their jobs allow it, and ask friends and family members for rides. It wasn't feasible to offer reduced service in the affected areas, Valley Metro said.

After negotiations unraveled over the past month, bus drivers on June 19 overwhelmingly voted to authorize a strike.

The mayors of Phoenix and Tempe called on the sides to quickly resolve their dispute.